1. Field of the Invention
This invention relates to a method for recycling monitoring control wafers, and more particularly, to a method for recycling monitoring control wafers used in an ion implanter.
2. Description of Related Art
During a semiconductor fabrication process, an ion implanter needs to be monitored periodically by a monitoring recipe to ensure that the ion implanter is functioning steadily.
Currently, there are two kinds of monitoring control wafers used in a medium-low current ion implanter. One is a bare silicon monitoring control wafer and the other is a monitoring control wafer with a screen layer. The process of using a bare silicon monitoring control wafer, using the measured sheet resistance (Rs) as the monitored target, in an ion implantation process is shown in FIG. 1. The monitoring process starts with taking a blank wafer and performing an ion implantation process by a monitoring recipe, then performing an annealing process such as a rapid thermal process (RTP), and then measuring the Rs. Measuring Rs is a destructive measurement because the measuring probe has to make contact with the surface of the control wafer. As a result, the only use for a measured bare silicon monitoring control wafer is as a dummy wafer. It can also be destroyed.
The other kind of control wafer is a control wafer with a screen layer and is used in an ion implantation process. The process of recycling the wafer that uses the measured thermal wave (TW) as the monitored target is shown in FIG. 2. The monitoring process starts with forming a screen layer on a control wafer, then measuring the TW value after performing an ion implantation process by a monitoring recipe, driving in implanted ions into the deeper areas of the screen layer, and then removing the screen layer and cleaning the control wafer. The control wafer can be reused as a recycled control wafer after forming a new screen layer on it.
Since the number of new bare silicon control wafers used for monitoring Rs is quite large in a fabrication line employing ion implanters, the cost is too high if these wafers are just treated as dummy wafers or destroyed after an implantation process.
Currently, a brand-new bare silicon wafer is still initially used as the control wafer in the process of monitoring TW value. The process starts with forming a screen layer on a brand-new bare silicon wafer, measuring the TW value after performing an implantation process, and then performing a recycling process, so that a certain cost is still required.